The invention relates to a sealing solution for the treatment of anodized aluminum or aluminum alloys comprising basically an aqueous solution, and relates also to a process for preventing deposits forming on the anodized surface during the sealing operation.
Anodizing is a surface treatment which is widely used with aluminum and its alloys. The oxide layer produced in this treatment considerably improves the resistance of the surface towards corrosive media. The natural or subsequent purposeful coloring of the oxide can also be used to provide a decorative finish to the part in question. One particularly important improvement from the technical point of view is that the oxide layer produced by anodizing increases the wear resistance of the surface of the aluminum.
There are various anodizing processes which can be used, depending on the properties required and the application in question. The most widely used of these is the so-called standard process employing direct current and solutions with sulphuric acid as the basis, and also another version in which oxalic acid is used.
There are also various processes for coloring the anodized surfaces, for example currentless coloring in solutions of organic or inorganic coloring agents, or electrolytic coloring with alternating current and solutions containing metal salts.
Oxide layers produced by anodizing do not, however, satisfy all requirements without some further treatment. The layers are porous, do not offer sufficient corrosion protection, and if colored, may have the colorant washed out of them again. The oxide, therefore, has to be sealed. This so called sealing process is usually carried out in hot or boiling water, if necessary with certain additions made to the water. As a result of this process, the pores are closed off, providing improved corrosion resistance and entrapping the colorant securely in the oxide.
The hydration of the aluminum oxide during the sealing process not only causes the pores to be closed off, but also results in the formation of a velvet so called sealing deposit on the surface. This deposit from the hydrated oxide impairs in particular the decorative appearance of dark colored oxide layers. Because the specific surface area is increased, this deposit also impairs the corrosion resistance and leads to discoloring of the surface. This sealing deposit is usually removed by mechanical polishing which of course entails extra labor and therefore considerable costs.
For some years now there have been processes which allow anodized surfaces to be sealed without the formation of a sealing deposit.
In the German patent DE-OS No. 14 46 461 for example a sealing solution containing basically nickel acetate and lignin sulphonate is described. It is also representative of the state of the art to prevent sealing deposits forming during the sealing process by using solutions which contain dextrin as an important constituent (DE-OS No. 19 44 452). The sealing bath in accordance with DE-OS No. 20 62 661 contains as the main constituents a water soluble nitrogen compound in combination with one or more of the substances dextrin, acrylic-acids or polyacrylate and lignin sulphonic acid. In the process disclosed in the patent DE-OS No. 21 08 725 the sealing bath used contains, besides nickel acetate and/or cobalt acetate, the ammonium salt of naphthaline sulphonic acid as a dispersion agent, sodium lauryl sulphate and octyl-phenoxy-polyethoxyethanol as wetting agent and malic acid as buffer. A further process makes use of an aqueous solution containing additions of oxy-carbonic acids or their salts (DE-OS No. 21 62 674).
The patent DE-OS No. 22 11 553 proposes a sealing bath containing, besides calcium ions, one or more water soluble phosphonic acids or their salts which form complexes with divalent metals. From patent DE-OS No. 22 07 681 it is also known to carry out the sealing process in a gelatine solution.
The sealing baths belonging to the present state of the art are characterized by way of a broad spectrum of additive substances. The reason for this wide variety of agents is that up to now it has not been possible to achieve sealing without hindrance from deposits, without impairing the quality of sealing achieved. Often the values of measurements which characterize the oxide layer were completely satisfactory when measured immediately after the sealing operation. Components of the sealing bath trapped in the pores of the oxide layers can however markedly affect the behavior of these layers over the long term; often it is not until months later that diminished corrosion resistance becomes apparent e.g. in the form of "chalking" of the previously deposit-free surface. Certain additives tend to produce a yellowing under the influence of light. Others remain partially as an adherent residue on the surface after drying; this then calls for a further rinsing step.
It is therefore an object of the invention to provide a sealing solution for anodically oxidized aluminum or aluminum alloys comprising basically an aqueous solution and a process which prevents the formation of deposits during the sealing of anodized aluminum, at the same time without having the above mentioned disadvantages.